Let's look at all the answers in turn:
Most people became farmers.
-this is false: industrial revolution pushed more people to be factory workers and not farmers
Most people moved to the countryside.
-this is also false: most people moved to cities, where work in factories was available.
America became more rural.
-this is again false, it became more urban instead
Population began to shift from the country to cities.
-this is the correct answer!
This shift had to do with the amount of work available in the cities.
